Offshore drilling rigs may operate at water depths exceeding 10,000 feet. When operating with a floating drilling unit (such as a drill ship or a semisubmersible drilling rig), the blowout preventers (BOPS) are generally located on the seafloor (rather than on the rig). The region between the BOP and the drilling rig is bridged by a series of large diameter tubes that are mechanically coupled to one another and make up the drilling riser. During a drilling operation the drill string is deployed in the drilling riser, with drilling fluid occupying the annular region between the drill string and the riser wall.
In a well control situation, formation fluids and/or gas can enter the well bore and may potentially result in a blowout if not properly controlled. The BOP commonly employs at least one mechanism for sealing the drill pipe in the event of formation fluid ingress. For example, pipe-rams may be used to seal against the drill-pipe. Some pipe-rams may preferably seal against the tubular section of the drill-pipe or are only able to seal against the tubular section of the drill-pipe, as they are specialized for such diameter.
In severe cases, in which sealing the drill pipe is inadequate, the final defense against a blowout may be to sever the drill pipe with a shear ram such as a blind shear ram (BSR) or a casing shear ram (CSR). These rams employ steel blades driven by hydraulic pistons to cut through the drill pipe and seal off the BOP bore. The rams and pistons are suitably strong to shear the tubular section of the drill pipe, but are not generally capable of shearing the drill pipe connections (located between the tubular sections) due to the significantly increased wall thickness of the connection. Thus, in the event that the drill pipe connection is located in the BSR or CSR, the drill pipe cannot be cut and the well cannot be properly sealed. There is therefore a need in the art for a method and apparatus capable of locating the drill pipe connections with respect to the BSR and CSR in a subsurface BOP.